Apparatus and method for beamforming in a multi-antenna system

ABSTRACT

An apparatus and method for beamforming in a multi-antenna system are provided. The method includes assuming at least one codeword comprised in a codebook as a precoding weight of a reference receive end and generating post-processing weights of at least two receive ends, confirming a codeword maximizing a sum rate using the post-processing weights, generating preceding weights of the receive ends using the codeword maximizing the sum rate, and precoding a transmit signal using the generated precoding weights and transmitting the precoded transmit signal to each receive end.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application claims priority under 35 U.S.C. §119(a) to aKorean Patent Application filed in the Korean Intellectual PropertyOffice on Feb. 4, 2008 and assigned Serial No. 10-2008-0011234, thecontents of which are herein incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present application relates generally to a multi-antenna system.More particularly, the present invention relates to an apparatus andmethod for performing coordinated beamforming in a multi-antenna systemof a multi-user environment.

BACKGROUND OF THE INVENTION

With a rapid growth of the wireless mobile communication market, ademand for a diversity of multimedia services in the wirelessenvironment is made. In order to provide multimedia services, a researchfor multi-antenna systems (e.g., Multiple Input Multiple Output (MIMO)systems) achieving a large capacity of transmit data and a high speed ofdata transmission while being able to efficiently utilize the limitedfrequency resources is made.

A multi-antenna system transmits data using an independent channel byantenna and thus, can increase transmission reliability and atransmission rate compared to a single antenna system even withoutadditional frequency or additional transmit power. Also, themulti-antenna system allows several users to simultaneously share spaceresources secured through a multiple antenna, thus being able to moreincrease frequency efficiency.

A multi-antenna system of a multiple user environment uses beamformingto cancel interference between users. For example, if the multi-antennasystem uses coordinated beamforming to cancel interference betweenusers, a transmit end generates a precoding weight and post-processingweight using downlink channel information. The precoding weight andpost-processing weight have vector or matrix values.

If the multi-antenna system uses coordinated beamforming as above, thetransmit end transmits a post-processing weight to a receive end. Thetransmit end transmits a post-processing weight to the receive end usinga dedicated pilot or a feedforward channel.

In the case of using a dedicated pilot, there is a problem that, becausethe transmit end allocates the dedicated pilot to avoid pilotsuperposition at each receive end, a pilot overhead increases.

In the case of using a feedforward channel, there is a problem that,because the transmit end quantizes a post-processing weight for eachreceive end and transmits the quantized post-processing weight over thefeedforward channel, a feedforward channel capacity increases inproportional to number of receive ends.

Also, in the case of reducing an amount of information transmitted overa feedforward channel from a transmit end, there is a problem that aquantization error increases and thus a throughput decreases.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is aprimary aspect of the present invention to substantially solve at leastthe above problems and/or disadvantages and to provide at least theadvantages below. Accordingly, one aspect of the present invention is toprovide an apparatus and method for generating a precoding weight andpost-processing weight for coordinated beamforming based on a codebookin a multi-antenna system of a multiple user environment.

Another aspect of the present invention is to provide an apparatus andmethod for generating a precoding weight and post-processing weight forcoordinated beamforming based on a codebook in a transmit end of amulti-antenna system of a multiple user environment.

A further aspect of the present invention is to provide an apparatus andmethod for generating post-processing weights using a referenceprecoding weight that is set based on a codebook in a transmit end of amulti-antenna system of a multiple user environment.

A further another aspect of the present invention is to provide anapparatus and method for generating a post-processing weight using acodeword received from a transmit end in a receive end of amulti-antenna system of a multiple user environment.

The above aspects are achieved by providing an apparatus and method forbeamforming in a multi-antenna system.

According to one aspect of the present invention, a method forbeamforming in a transmit end of a multi-antenna system is provided. Themethod includes assuming each codeword in a codebook as a precodingweight of a reference receive end and generating post-processing weightsof at least two receive ends, calculating a sum rate of each codewordwhich assumed the precoding weight of the reference receive end, usingthe post-processing weights, comparing the sum rate for each codewordand selecting a codeword maximizing a sum rate using the post-processingweights, generating preceding weights of the receive ends using thecodeword maximizing the sum rate, and preceding a transmit signal usingthe generated precoding weights and transmitting the precoded transmitsignal to each receive end, wherein the codebook includes at least onecodeword, the reference receive end is among at least two receive endfor service, the at least two receive includes the reference receiveend.

According to another aspect of the present invention, a method fordetecting a signal in a receive end of a multi-antenna system isprovided. The method includes estimating a channel using signalsreceived through at least two antennas, confirming a codewordcorresponding to codeword index information received over a feedforwardchannel in a codebook comprising at least one codeword, generating apost-processing weight using the confirmed codeword and the estimatedchannel, estimating a channel gain using the estimated channel and thepost-processing weight, and detecting a signal using the post-processingweight and the channel gain.

According to a further aspect of the present invention, an apparatus forbeamforming in a transmit end of a multi-antenna system is provided. Theapparatus includes a channel confirmer, a codeword selector, a weightgenerator, and a precoder. The channel confirmer confirms channelinformation of at least two receive ends. The codeword selector assumeseach codeword in a codebook as a precoding weight of a reference receiveend and generates post-processing weights of at least two receive endsusing the codeword, selects a codeword maximizing a sum rate using thepost-processing weights for each codeword. The weight generatorgenerates precoding weights for the receive ends using the codewordmaximizing the sum rate. The precoder precodes a transmit signal usingthe generated precoding weights and transmits the precoded transmitsignal to each receive end.

According to a yet another aspect of the present invention, a receiveend apparatus of a multi-antenna system is provided. The apparatusincludes at least two antennas, a channel estimator, a codewordconfirmer, a weight generator, a channel gain estimator, and apost-processor. The at least two antennas receive signals. The channelestimator estimates a channel using the signals received through theantennas. The codeword confirmer confirms a codeword corresponding tocodeword index information received over a feedforward channel in acodebook comprising at least one codeword. The weight generatorgenerates a post-processing weight using the confirmed codeword and theestimated channel. The channel gain estimator estimates a channel gainusing the channel estimated in the channel estimator and thepost-processing weight generated in the weight generator. Thepost-processor post-processes signals received through the antennasusing the post-processing weight and channel gain.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 is a flow diagram illustrating a process of generating apreceding weight and post-processing weight for coordinated beamformingin a transmit end of a multi-antenna system according to an exemplaryembodiment of the present invention;

FIG. 2 is a flow diagram illustrating a process of confirming apost-processing weight for coordinated beamforming in a receive end of amulti-antenna system according to an exemplary embodiment of the presentinvention;

FIG. 3 is a block diagram illustrating a construction of a transmit endin a multi-antenna system according to an exemplary embodiment of thepresent invention; and

FIG. 4 is a block diagram illustrating a construction of a receive endin a multi-antenna system according to an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 4, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication network.

A technology for generating a preceding weight and post-processingweight for coordinated beamforming based on a codebook in amulti-antenna system of a multiple user environment according to anexemplary embodiment of the present invention is described below.

In the following description, the precoding weight is called a transmitbeamforming weight, and the post-processing weight is called a receivebeamforming weight.

It is assumed that a transmit end is aware of downlink channelinformation of receive ends located in a service area. However, the sameis applicable even if the transmit end is aware of inaccurate downlinkchannel information. If using a Time Division Duplex (TDD) scheme, atransmit end can estimate a downlink channel through a sounding channelor confirm downlink channel information estimated by respective receiveends through a feedback channel.

It is also assumed that a transmit end and a receive end share the samecodebook to generate a transmit beamforming weight and a receivebeamforming weight.

As shown in FIG. 1 below, a transmit end of a multi-antenna systemgenerates a transmit beamforming weight and receive beamforming weightfor coordinated beamforming based on a codebook.

FIG. 1 is a flow diagram illustrating a process of generating aprecoding weight and post-processing weight for coordinated beamformingin a transmit end of a multi-antenna system according to an exemplaryembodiment of the present invention.

Referring to FIG. 1, in step 101, the transmit end generates receivebeamforming weights for receive ends using an m^(th) codeword of acodebook. The transmit end generates receive beamforming weights of allreceive ends for providing service using the m^(th) codeword. Forexample, when the m^(th) codeword is a transmit beamforming weight for aj^(th) receive end, the transmit end generates receive beamformingweights for receive ends as given in Equation 1 below. The transmit endsets the transmit beamforming weight of the j^(th) receive end as areference transmit beamforming weight.

$\begin{matrix}{R_{k} = \left\{ {{\begin{matrix}{{\left( {H_{k}c_{m}} \right)^{H}/{{H_{k}c_{m}}}},{k = j}} \\{{f_{\bot}\left( {H_{k}c_{m}} \right)},{otherwise}}\end{matrix}{where}\mspace{14mu} {f_{\bot}\left( \begin{bmatrix}\alpha \\\beta\end{bmatrix} \right)}} = {{\frac{1}{\sqrt{{\alpha }^{2} + {\beta }^{2}}}\left\lbrack {\beta - \alpha} \right\rbrack}.}} \right.} & \left\lbrack {{Eqn}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, ‘R_(k)’ denotes a normalized receive beamforming weightfor a k^(th) receive end, ‘H_(k)’ denotes downlink channel informationof the k^(th) receive end, ‘C’ denotes a codebook, and ‘c_(m)’ denotesan m^(th) codeword of the codebook. The value ‘c_(m)’ means a transmitbeamforming weight for a receive end having a reference transmitbeamforming weight.

The codebook (C) includes codes of 2^(M) depending on number (M) of bitsconstituting the codebook (C={c₁, c₃, . . . , c₂ _(M) }).

In Equation 1, the transmit end sets an m^(th) codeword as a transmitbeamforming weight of a j^(th) receive end. Thus, the transmit endgenerates a matched filter for the product of a channel of the j^(th)receive end and the m^(th) codeword and generates a receive beamformingweight of the j^(th) receive end. The matched filter means a filter forcarrying out a conjugate transpose operation for a channel vector thatis comprised of the product of a codeword and a channel.

If generating receive beamforming weights for other receive ends thanthe j^(th) receive end, the transmit end selects an orthogonal vectorfor the product of a channel of a receive end for generating a receivebeamforming weight and the m^(th) codeword. After that, the transmit endgenerates a matched filter for the orthogonal vector and generates areceive beamforming weight of the receive end. The transmit endnormalizes the generated receive beamforming weight to have a unit norm.

After generating the receive beamforming weights of the receive endsusing the m^(th) codeword in step 101, the transmit end proceeds to step103 and confirms if it generates receive beamforming weights of receiveends using all codewords included in a codebook. Thus, the transmit endcompares an index (m) of the codeword used to generate the receivebeamforming weight in step 101 with the total number (M) of thecodewords included in the codebook.

If ‘m’ is not equal to ‘M’ (m≠M) in step 103, the transmit enddetermines that it does not generate the receive beamforming weights ofthe receive ends using all the codewords. If so, in step 111, thetransmit end increases a codeword index by one level (m++).

After increasing the codeword index by one level, the transmit endreturns to step 101 and generates receive beamforming weights forreceive ends using the m^(th) codeword.

If ‘m’ is equal to ‘M’ (m=M) in step 103, the transmit end determinesthat it generates the receive beamforming weights of the receive endsusing all the codewords. If so, in step 105, the transmit end selects acodeword maximizing a sum rate among the codewords included in thecodebook. For example, the transmit end selects a codeword maximizing asum rate using Equation 2 below. The transmit end selects a codewordmaximizing a sum rate using an effective channel (G), as given inEquation 2 below. However, the transmit end may select a codewordmaximizing a different parameter by controlling a transmit power.

$\begin{matrix}{{c_{opt} = {\begin{matrix}{\arg \; \min} \\{c_{m} \in C}\end{matrix}{{Tr}\left( \left( {GG}^{H} \right)^{- 1} \right)}\mspace{14mu} {however}}},{G = {\begin{bmatrix}R_{1} & \; & H_{1} \\\; & \vdots & \; \\R_{K} & \; & H_{K}\end{bmatrix}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, ‘c_(opt)’ denotes a codeword maximizing a sum rate,‘c_(m)’ denotes an m^(th) codeword in a codebook, ‘G’ denotes aneffective channel matrix that is a channel matrix applying a receivebeamforming weight, ‘R_(k)’ denotes a normalized receive beamformingweight for a k^(th) receive end, and ‘H_(k)’ denotes downlink channelinformation of the k^(th) receive end.

In Equation 2, the codeword maximizing the sum rate signifies a codewordminimizing an operation for Tr((GG^(H))⁻¹) among the codewords includedin the codebook. That is, if the transmit end uses an inverse matrix(G⁻¹) of an effective channel (G) as a transmit beamforming weightmatrix, receive ends can receive signals without interference. If thetransmit end does not separately normalize a transmit power of thetransmit end by receive end, the transmit end divides a transmit signalby the Frobenius norm for the inverse matrix of the effective channelfor transmission to normalize the whole signal. As the Frobenius normvalue decreases, a Signal to Noise Ratio (SNR) of a receive endreceiving a transmit signal increases. Thus, the transmit end selects acodeword minimizing an operation for Tr((GG^(H))⁻¹) as in Equation 2.The Frobenius norm means a square root for a sum of the product of allelements of a matrix.

The transmit beamforming weight of Equation 2 is not normalized. Ifnormalizing a transmit beamforming weight, the transmit end selects acodeword maximizing a sum rate using Equation 3:

$\begin{matrix}{c_{opt} = {\begin{matrix}{\arg \; \min} \\{c_{i} \in C}\end{matrix}{\sum\limits_{k}{{\log \left( {1 + {{{H_{k}T_{k}}}^{2} \cdot \frac{E_{s}}{{KN}_{0}}}} \right)}.}}}} & \left\lbrack {{Eqn}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

In Equation 3, ‘c_(opt)’ denotes a codeword maximizing a sum rate,‘c_(i)’ denotes an i^(th) codeword in a codebook, ‘H_(k)’ denotesdownlink channel information of a k^(th) receive end, ‘E_(s)’ denotesthe whole transmit power, ‘K’ denotes number of receive ends, ‘N₀’denotes a magnitude of a noise power of a receive end, ‘T_(k)’ denotes atransmit beamforming weight for the k^(th) receive end, and ‘C’ denotesa codebook.

The transmit end normalizes and calculates the transmit beamformingweight for the k^(th) receive end as given in Equation 4:

$\begin{matrix}{{T_{k} = \frac{q_{k}}{q_{k}}},{\left\lbrack {q_{1},\ldots \mspace{14mu},q_{K}} \right\rbrack = {\begin{bmatrix}R_{1}^{H} & \; & H_{1} \\\; & \vdots & \; \\R_{K}^{H} & \; & H_{K}\end{bmatrix}^{- 1}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, ‘T_(k)’ denotes a normalized transmit beamforming weightfor a k^(th) receive end, ‘q_(k)’ denotes an effective channel applyinga receive beamforming weight, ‘R_(k)’ denotes a receive beamformingweight for the k^(th) receive end, and ‘H_(k)’ denotes downlink channelinformation of the k^(th) receive end.

In Equation 3, the codeword maximizing the sum rate signifies a codewordwhose sum rate is maximized. That is, if the transmit end normalizes anduses each column of an inverse matrix (G⁻¹) of an effective channel (G)as a transmit beamforming weight matrix, receive ends can receivesignals without interference.

After determining the codeword maximizing the sum rate in step 105, thetransmit end goes to step 107 and generates transmit beamforming weightsfor receive ends using the codeword maximizing the sum rate. Assumingtransmitting a signal to one receive end through one stream, thetransmit end generates transmit beamforming weights for receive ends byperforming power normalization as in Equation 5 below such that streamseach have the same SNR.

$\begin{matrix}{T = {\frac{G^{- 1}}{{Tr}\left( \left( {GG}^{H} \right)^{- 1} \right)}.}} & \left\lbrack {{Eqn}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

In Equation 5, ‘T’ denotes a matrix of transmit beamforming weights forreceive ends, and ‘G’ denotes an effective channel matrix applying areceive beamforming weight.

As in Equation 5, the transmit end normalizes a transmit signal for aninverse matrix of an effective channel and generates a transmitbeamforming weight. Column vectors of the ‘T’ matrix represent transmitbeamforming weights of data to be transmitted to respective receiveends.

As described above, the transmit end generates transmit beamformingweights using a codeword maximizing a sum rate. The transmit endrecognizes, as receive beamforming weights of receive ends, receivebeamforming weights generated using the codeword maximizing the sum rateamong the receive beamforming weights generated using the codeword instep 101.

Unlike Equation 5, the transmit end may normalize each column of aninverse matrix (G⁻¹) and generate transmit beamforming weights as inEquation 6 below. The transmit end normalizes each column to 1/sqrt (K),not ‘1’, to set the whole power to ‘1’.

$\begin{matrix}{{T = {\frac{1}{\sqrt{K}}\left\lbrack {T_{1},\ldots \mspace{14mu},T_{k}} \right\rbrack}}{T_{k} = {{\frac{q_{k}}{q_{k}}\left\lbrack {q_{1},\ldots \mspace{14mu},q_{K}} \right\rbrack} = {\begin{bmatrix}R_{1}^{H} & \; & H_{1} \\\; & \vdots & \; \\R_{K}^{H} & \; & H_{K}\end{bmatrix}^{- 1}.}}}} & \left\lbrack {{Eqn}.\mspace{14mu} 6} \right\rbrack\end{matrix}$

In Equation 6, ‘T’ denotes a matrix of transmit beamforming weights forreceive ends, ‘K’ denotes an index of a receive end, ‘T_(k)’ denotes anormalized transmit beamforming weight for a k^(th) receive end, ‘q_(k)’denotes an effective channel applying a receive beamforming weight,‘R_(k)’ denotes a receive beamforming weight for the k^(th) receive end,and ‘H_(k)’ denotes downlink channel information of the k^(th) receiveend.

After generating the transmit beamforming weights in step 107, thetransmit end goes to step 109 and performs beamforming to acorresponding receive end through each antenna using the transmitbeamforming weight generated in step 107. At this time, the transmit endtransmits an index of a codeword maximizing a sum rate to receive endsover a feedforward channel.

The transmit end transmits one pilot shared by at least one or moretransmit beamforming weights among transmit beamforming weights.

After that, the transmit end terminates the process according to anexemplary embodiment of the present invention.

In the aforementioned exemplary embodiment of the present invention, atransmit end normalizes power as in Equation 5 or normalizes power byreceive end as in Equation 6 in order to set the same SNR to respectivestreams.

In another exemplary embodiment of the present invention, if setting adifferent SNR to each stream, a transmit end may multiply each stream bya power loading value using a diagonal matrix.

As described above, as given in Equation 1, a transmit end sets acodeword included in a codebook as a transmit beamforming weight of aj^(th) receive end and generates a receive beamforming weight for eachreceive end. Thus, the receive end can generate a receive beamformingweight using only information on the codeword received from the transmitend as shown in FIG. 2 below.

FIG. 2 is a flow diagram illustrating a process of confirming apost-processing weight for coordinated beamforming in a receive end of amulti-antenna system according to an exemplary embodiment of the presentinvention.

Referring to FIG. 2, in step 201, the receive end confirms a codeword(c_(opt)) maximizing a sum rate among codewords included in a codebook.For example, the receive end confirms, in the codebook, a codeword(c_(opt)) corresponding to a codeword index received through afeedforward channel.

After confirming the codeword maximizing the sum rate, the receive endgoes to step 203 and generates a receive beamforming weight using thecodeword and channel information. For example, the receive end generatesa receive beamforming weight as given in Equation 7 below. The receiveend normalizes the receive beamforming weight to have a unit norm.

$\begin{matrix}{R_{k} = \left\{ {{\begin{matrix}{{\left( {H_{k}c_{opt}} \right)^{H}/{{H_{k}c_{opt}}}},{k = j}} \\{{f_{\bot}\left( {H_{k}c_{opt}} \right)},{otherwise}}\end{matrix}{where}\mspace{14mu} {f_{\bot}\left( \begin{bmatrix}\alpha \\\beta\end{bmatrix} \right)}} = {{\frac{1}{\sqrt{{\alpha }^{2} + {\beta }^{2}}}\left\lbrack {\beta - \alpha} \right\rbrack}.}} \right.} & \left\lbrack {{Eqn}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

In Equation 7, ‘R_(k)’ denotes a normalized receive beamforming weightfor a k^(th) receive end, ‘H_(k)’ denotes downlink channel informationof the k^(th) receive end, and ‘c_(opt)’ denotes a codeword maximizing asum rate.

As in Equation 7, the receive end generates a matched filter for theproduct of a downlink channel with a transmit end and a codewordmaximizing a sum rate and generates a receive beamforming weight.

After generating the receive beamforming weight in step 203, the receiveend goes to step 205 and calculates an effective channel gain using thereceive beamforming weight generated in step 203. For example, thereceive end estimates an effective channel gain using the generatedreceive beamforming weight and the estimated channel.

After acquiring the receive beamforming weight and effective channelgain, in step 207, the receive end detects a signal using the receivebeamforming weight and effective channel gain.

Then, the receive end terminates the process according to an exemplaryembodiment of the present invention.

The following description is for a method for generating andtransmitting a receive beamforming weight in a base station (BS) of amulti-antenna system that is comprised of a BS including two antennasand two mobile stations (MSs) including two antennas. The assumption isthat one MS receives a signal through one stream from the BS.

As in Equation 8 below, the BS generates receive beamforming weights ofan MS 1 and an MS 2 using an m^(th) codeword. It is assumed that atransmit beamforming weight of the MS 1 is set as a reference transmitbeamforming weight.

$\begin{matrix}{{R_{1} = {\left( {H_{1}c_{m}} \right)^{H}/{{H_{1}c_{m}}}}}{R_{2} = {{f_{\bot}\left( {H_{2}c_{m}} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 8} \right\rbrack\end{matrix}$

In Equation 8, ‘R_(k)’ denotes a normalized receive beamforming weightfor a k^(th) receive end, ‘H_(k)’ denotes downlink channel informationof the k^(th) receive end, and ‘c_(m)’ denotes an m^(th) codeword of acodebook. The value ‘c_(m)’ means a transmit beamforming weight of afirst MS having a reference transmit beamforming weight.

The BS constructs an effective channel matrix using the receivebeamforming weights of MSs generated in Equation 8, as given in Equation9:

$\begin{matrix}{G = {\begin{bmatrix}R_{1} & H_{1} \\R_{2} & H_{2}\end{bmatrix}.}} & \left\lbrack {{Eqn}.\mspace{14mu} 9} \right\rbrack\end{matrix}$

In Equation 9, ‘G’ denotes an effective channel matrix applying areceive beamforming weight, ‘R_(k)’ denotes a normalized receivebeamforming weight for a k^(th) MS, and ‘H_(k)’ denotes downlink channelinformation of the k^(th) MS.

If a codebook is constituted of two bits, the BS generates foureffective channel matrixes constructed as in Equation 9. The BS appliesthe effective channel matrixes to Equation 2 and determines a codewordhaving the maximum sum rate. For another example, the BS may determine acodeword having the maximum sum rate using Equation 3.

The BS applies an effective matrix of a codeword having the maximum sumrate to Equation 5 and generates transmit beamforming weights for MSs.For another example, the BS may generate transmit beamforming weightsfor MSs using Equation 6. The BS transmits an index of a codeword havingthe maximum sum rate to an MS over a feedforward channel.

If the BS transmits an index of a codeword over a feedforward channel asabove, an MS generates a receive beamforming weight according to thecodeword index received from the BS as in Equation 10:

$\begin{matrix}{{R_{1} = {\left( {H_{1}c_{opt}} \right)^{H}/{{H_{1}c_{opt}}}}}{R_{2} = {{f_{\bot}\left( {H_{2}c_{opt}} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, ‘R_(k)’ denotes a receive beamforming weight for ak^(th) MS, ‘H_(k)’ denotes downlink channel information of the k^(th)MS, and ‘c_(opt)’ denotes a codeword acquired through a feedforwardchannel.

In the aforementioned exemplary embodiment of the present invention, atransmit end sets a codeword of a codebook as a transmit beamformingweight of a first receive end and generates transmit beamforming weightsand receive beamforming weights for other receive ends.

In another exemplary embodiment of the present invention, a transmit endmay generate transmit beamforming weights and receive beamformingweights for other receive ends on the basis of a transmit beamformingweight having the highest sum rate without setting a codeword of acodebook as a transmit beamforming weight of a specific receive end. Forexample, as given Equation 11 below, the transmit end generates transmitbeamforming weights and receive beamforming weights for receive ends onthe basis of a transmit beamforming weight having the highest sum rate.The transmit end can simultaneously determine a reference transmitbeamforming weight and a codeword generating the reference transmitbeamforming weight.

$\begin{matrix}{\left\{ {p_{opt},c_{opt}} \right\} = {\begin{matrix}{\arg \; \min} \\\left\{ {{1 \leq p \leq K},{c_{i} \in C}} \right\}\end{matrix}{{{Tr}\left( \left( {GG}^{H} \right)^{- 1} \right)}.}}} & \left\lbrack {{Eqn}.\mspace{14mu} 11} \right\rbrack\end{matrix}$

In Equation 11, ‘p_(opt)’ denotes an index of a transmit beamformingweight having the highest sum rate, ‘c_(opt)’ denotes an index of acodeword for a transmit beamforming weight having the highest sum rate,‘G’ denotes an effective channel matrix applying a receive beamformingweight, ‘c_(i)’ denotes an i^(th) codeword included in a codebook (C),and ‘K’ denotes number of receive ends. The index of the transmitbeamforming weight is the same as an index of a receive end for thetransmit beamforming weight.

Assuming that ‘p’ denotes an index of a reference transmit beamformingweight of Equation 11, a transmit end generates a receive beamformingweight on the basis of a transmit beamforming weight of a p^(th) receiveend as in Equation 12:

$\begin{matrix}{R_{k} = \left\{ \begin{matrix}{{\left( {H_{k}c_{m}} \right)^{H}/{{H_{k}c_{m}}}},{k = p}} \\{{f_{\bot}\left( {H_{k}c_{m}} \right)},{{otherwise}.}}\end{matrix} \right.} & \left\lbrack {{Eqn}.\mspace{14mu} 12} \right\rbrack\end{matrix}$

In Equation 12, ‘R_(k)’ denotes a normalized receive beamforming weightfor a k^(th) receive end, ‘H_(k)’ denotes downlink channel informationof the k^(th) receive end, ‘C’ denotes a codebook, and ‘c_(m)’ denotesan m^(th) codeword of the codebook.

The following description is for a construction of a transmit end forgenerating a precoding weight and post-processing weight for coordinatedbeamforming based on a codebook.

FIG. 3 is a block diagram illustrating a construction of a transmit endin a multi-antenna system according to an exemplary embodiment of thepresent invention.

As shown in FIG. 3, the transmit end includes encoders 300-1, 300-2, and300-N_(T), modulators 310-1, 310-2, and 310-N_(T), a precoder 320, RadioFrequency (RF) processors 330-1, 330-2, and 330-N_(T), a weightgenerator 340, a codeword and pilot selector 350, and a channelconfirmer 360.

The encoders 300-1, 300-2, and 300-N_(T) are constructed by antenna andencode transmit data to be transmitted over a corresponding antennaaccording to a corresponding modulation level (e.g., a Modulation andCoding Scheme (MCS) level). The modulators 310-1, 310-2, and 310-N_(T)are constructed by antenna and modulate signals received from thecorresponding encoders 300-1, 300-2, and 300-N_(T) according to acorresponding modulation level.

The precoder 320 precodes data received from the respective modulators310-1, 310-2, and 310-N_(T) using a transmit beamforming weight receivedfrom the weight generator 340. The precoder 320 outputs data to betransmitted through a first antenna (N₁) to the first RF processor330-1, and outputs data to be transmitted through an N_(T) ^(th) antenna(N_(T)) to an N_(T) ^(th) RF processor 330-N_(T).

The precoder 320 transmits one pilot shared by at least one or moretransmit beamforming weights among the transmit beamforming weights.

The RF processors 330-1, 330-2, and 330-N_(T) convert data received fromthe precoder 320 into analog RF signals and transmit the convertedsignals to respective receive ends through corresponding antennas (N₁,N₂, and N_(T)).

The channel confirmer 360 confirms channels of receive ends located in aservice area. For example, the channel confirmer 360 estimates a channelthrough a sounding channel. In another example, the channel confirmer360 confirms channel information received over a feedback channel fromreceive ends.

The codeword and pilot selector 350 generates receive beamformingweights of receive ends for providing service using channel informationconfirmed in the channel confirmer 360 and codewords included in acodebook. Although not illustrated, the codeword and pilot selector 350is comprised of a receive beamforming weight generator and a codewordselector. The beamforming weight generator generates receive beamformingweights of receive ends using Equation 1. The codeword selector appliesan effective channel matrix of receive beamforming weights generated inthe beamforming weight generator to Equation 2 and determines a codewordmaximizing a sum rate. The codeword selector may determine a codewordmaximizing a sum rate using Equation 3.

In yet another example, the codeword and pilot selector 350 selects acodeword having the highest sum rate and a receive end having areference transmit beamforming weight using Equation 11. After that, thecodeword and pilot selector 350 generates receive beamforming weightsfor receive ends using Equation 12 on the basis of the selected receiveend.

The weight generator 340 generates transmit beamforming weights forreceive ends by performing power normalization of Equation 5 such thatstreams each have the same SNR using a codeword selected in the codewordand pilot selector 350. Column vectors of a matrix generated as inEquation 5 represent transmit beamforming weights of respective receiveends.

In still another example, the weight generator 340 may generate transmitbeamforming weights for receive ends by normalizing each column of aninverse matrix (G⁻¹) as in Equation 6. The transmit end normalizes eachcolumn to 1/sqrt (K), not ‘1’, to set the whole power to ‘1’.

Although not illustrated, the transmit end includes a control channeltransmitter, and transmits index information of a codeword maximizing asum rate determined in the codeword and pilot selector 350 to receiveends over a feedforward channel. The index information of the codewordrepresents an index of a codeword included in a codebook.

The following description is for a construction of a receive end forgenerating a post-processing weight for coordinated beamforming based ona codebook.

FIG. 4 is a block diagram illustrating a construction of a receive endin a multi-antenna system according to an exemplary embodiment of thepresent invention.

As shown in FIG. 4, the receive end includes RF processors 400-1, 400-2,and 400-N_(R), a post-processor 410, a demodulator 420, a decoder 430, achannel confirmer 440, a weight generator 450, and a codeword and pilotconfirmer 460.

The RF processors 400-1, 400-2, and 400-N_(R) convert high frequencysignals received through respective antennas into baseband signals.

The post-processor 410 post-processes signals received from therespective RF processors 400-1, 400-2, and 400-N_(R) using a receivebeamforming weight received from the weight generator 450.

The demodulator 420 demodulates a signal received from thepost-processor 410 according to a corresponding modulation level. Thedecoder 430 decodes a signal received from the demodulator 420 accordingto a corresponding modulation level and detects original data.

The channel confirmer 440 estimates a channel with a transmit end usingpilots included in signals received from the RF processors 400-1, 400-2,and 400-N_(R). For example, if the transmit end transmits one pilotshared by at least one or more transmit beamforming weights, the channelconfirmer 440 estimates a channel using the pilot.

The codeword and pilot confirmer 460 confirms a codeword correspondingto index information of a codeword received from the transmit end over afeedforward channel, in a codebook.

The codeword and pilot confirmer 460 acquires information of a referencereceive end selected to generate a transmit beamforming weight and areceive beamforming weight in the transmit end.

The weight generator 450 generates a receive beamforming weight forpost-processing a signal in the post-processor 410 using a codewordreceived from the codeword and pilot confirmer 460. For example, theweight generator 450 generates a receive beamforming weight using thecodeword as in Equation 7. The weight generator 450 generates a receivebeamforming weight using Equation 7 depending on reference receive endinformation confirmed in the codeword and pilot confirmer 460.

Although not illustrated, the receive end further includes a channelgain estimator. The channel gain estimator estimates a channel gainusing a channel estimated in the channel confirmer 440 and apost-processing weight generated in the weight generator 450.

Thus, the post-processor 410 may post-process signals received from therespective RF processors 400-1, 400-2, 400-N_(R) using a receivebeamforming weight received from the weight generator 450 and a channelgain received from the channel gain estimator.

As described above, an exemplary embodiment of the present invention hasan advantage of, by generating a precoding weight and post-processingweight based on a codebook, generating the precoding weight andpost-processing weight without performing a repeated operation, thusreducing an operation complexity and reducing a feedforward informationamount transmitting the post-processing weight in a multi-antenna systemof a multiple user environment.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. A method for beamforming in a transmit end of a multi-antenna system,the method comprising: assuming each codeword in a codebook as aprecoding weight of a reference receive end and generatingpost-processing weights of at least two receive ends using the codeword;calculating a sum rate for each codeword which assumed the precedingweight of the reference receive end, using the post-processing weights;comparing the sum rate for each codeword and selecting a codewordmaximizing a sum rate; generating preceding weights of the receive endsusing the codeword maximizing the sum rate; and precoding a transmitsignal using the generated precoding weights, and transmitting theprecoded transmit signal to each receive end; wherein the codebookincludes at least one codeword; the reference receive end is among atleast two receive end for service; the at least two receive end includesthe reference receive end.
 2. The method of claim 1, wherein generatingthe post-processing weight comprises: confirming channel information ofthe reference receive end; and generating a channel vector for theproduct of the selected channel information of the reference receive endand the codeword assumed as the precoding weight of the referencereceive end; and operating a conjugate transpose for the channel vectorand generating a post-processing weight.
 3. The method of claim 1,wherein generating the post-processing weight comprises: confirmingchannel information of a receive end, not the reference receive end;confirming an orthogonal vector for the product of the confirmed channelinformation of the receive end and the codeword assumed as the precodingweight of the reference receive end; and operating a conjugate transposefor the orthogonal vector and generating a post-processing weight. 4.The method of claim 1, wherein confirming the codeword maximizing thesum rate comprises: generating an effective channel using thepost-processing weight for the codeword assumed as the preceding weightof the reference receive end; and calculating a sum rate of eachcodeword which assumed the precoding weight of the reference receiveend, using the effective channel.
 5. The method of claim 1, wherein thecodeword maximizing the sum rate is selected using a first equation, thefirst equation defined as: ${c_{opt} = {\begin{matrix}{\arg \; \min} \\{c_{m} \in C}\end{matrix}{{Tr}\left( \left( {GG}^{H} \right)^{- 1} \right)}\mspace{14mu} {however}}},{G = \begin{bmatrix}R_{1} & \; & H_{1} \\\; & \vdots & \; \\R_{K} & \; & H_{K}\end{bmatrix}},$ wherein c_(opt): codeword maximizing sum rate, c_(m):m^(th) codeword in codebook, and G: effective channel matrix that ischannel matrix applying post-processing weight.
 6. The method of claim1, wherein the codeword maximizing the sum rate is selected using asecond equation, the second equation defined as:$c_{opt} = {\begin{matrix}{\arg \; \min} \\{c_{i} \in C}\end{matrix}{\sum\limits_{k}{\log \left( {1 + {{{H_{k}T_{k}}}^{2} \cdot \frac{E_{s}}{{KN}_{0}}}} \right)}}}$${T_{k} = \frac{q_{k}}{q_{k}}},{\left\lbrack {q_{1},\ldots \mspace{14mu},q_{K}} \right\rbrack = \begin{bmatrix}R_{1}^{H} & \; & H_{1} \\\; & \vdots & \; \\R_{K}^{H} & \; & H_{K}\end{bmatrix}^{- 1}},$ wherein c_(opt): codeword maximizing sum rate,c_(i): i^(th) codeword in codebook, H_(k): downlink channel informationof k^(th) receive end, E_(s): whole transmit power, K: number of receiveends, N₀: magnitude of noise power of receive end, T_(k): precodingweight for k^(th) receive end, q_(k): effective channel applyingpost-processing weight, and R_(k): post-processing weight for k^(th)receive end.
 7. The method of claim 1, further comprising, afterconfirming the codeword maximizing the sum rate, transmitting an indexof the codeword maximizing the sum rate to the receive ends over afeedforward channel.
 8. The method of claim 1, further comprising, aftergenerating the preceding weights, transmitting a pilot shared by atleast one or more precoding weights among the precoding weights.
 9. Amethod for detecting a signal in a receive end of a multi-antennasystem, the method comprising: estimating a channel using signalsreceived through at least two antennas; confirming a codewordcorresponding to codeword index information received over a feedforwardchannel in a codebook comprising at least one codeword; generating apost-processing weight using the confirmed codeword and the estimatedchannel; estimating a channel gain using the estimated channel and thepost-processing weight; and detecting a signal using the post-processingweight and the channel gain.
 10. The method of claim 9, wherein, if atransmit end sets the receive end as a reference receive end, generatingthe post-processing weight comprises: generating a channel vector forthe product of the confirmed codeword and the estimated channel; andoperating a conjugate transpose for the channel vector and generating apost-processing weight.
 11. The method of claim 9, wherein, if atransmit end does not set the receive end as a reference receive end,generating the post-processing weight comprises: confirming anorthogonal vector for the product of the confirmed codeword and theestimated channel; and operating a conjugate transpose for theorthogonal vector and generating a post-processing weight.
 12. Anapparatus for beamforming in a transmit end of a multi-antenna system,the apparatus comprising: a channel confirmer for confirming channelinformation of at least two receive ends; a codeword selector forassuming each codeword in a codebook as a preceding weight of areference receive end and generating post-processing weights of at leasttwo receive ends using the codeword, selecting a codeword maximizing asum rate using the post-processing weights for each codeword; a weightgenerator for generating precoding weights for the receive ends usingthe codeword maximizing the sum rate; and a precoder for precoding atransmit signal using the generated precoding weights and transmittingthe precoded transmit signal to each receive end; wherein the codebookincludes at least one codeword; the reference receive end is among atleast two receive end for service.
 13. The apparatus of claim 12,wherein the codeword selector comprises: a post-processing weightgenerator for assuming each codeword in the codebook as a precodingweight of a reference receive end, generating post-processing weightsfor at least two receive ends for providing service; and a codewordselector for selecting a codeword maximizing a sum rate using thepost-processing weights generated in the post-processing weightgenerator.
 14. The apparatus of claim 13, wherein the post-processingweight generator generates a channel vector for the product of theselected channel information of the reference receive end and thecodeword assumed as the precoding weight of the reference receive end,operating a conjugate transpose for the channel vector, and generating apost-processing weight.
 15. The apparatus of claim 13, wherein thepost-processing weight generator confirms an orthogonal vector for aproduct of channel information of a receive end, not the referencereceive end and the codeword assumed as the precoding weight of thereference receive end, generates operating a conjugate transpose for theorthogonal vector, and generates a post-processing weight.
 16. Theapparatus of claim 13, wherein the codeword selector selects a codewordmaximizing a sum rate using an effective channel generated using thepost-processing weights for the codeword assumed as the preceding weightof the reference receive end.
 17. The apparatus of claim 12, wherein theprecoder transmits one pilot shared by at least one or more precodingweights among the precoding weights generated in the weight generator.18. The apparatus of claim 12, further comprising a control channeltransmitter for transmitting an index of the codeword maximizing the sumrate to the receive ends over a feedforward channel.
 19. A receive endapparatus of a multi-antenna system, the apparatus comprising: at leasttwo antennas for receiving signals; a channel estimator for estimating achannel using the signals received through the antennas; a codewordconfirmer for confirming a codeword corresponding to codeword indexinformation received over a feedforward channel in a codebook comprisingat least one codeword; a weight generator for generating apost-processing weight using the confirmed codeword and the estimatedchannel; a channel gain estimator for estimating a channel gain usingthe channel estimated in the channel estimator and the post-processingweight generated in the weight generator; and a post-processor forpost-processing signals received through the antennas using thepost-processing weight and channel gain.
 20. The apparatus of claim 19,wherein, if a transmit end sets a receive end as a reference receiveend, the weight generator generates a channel vector for the product ofthe channel information estimated in the channel estimator and thecodeword confirmed in the codeword confirmer, operating a conjugatetranspose for the channel vector, and generating a post-processingweight.
 21. The apparatus of claim 19, wherein, if a transmit end doesnot set a receive end as a reference receive end, the weight generatorconfirms an orthogonal vector for the product of the channel informationestimated in the channel estimator and the codeword confirmed in thecodeword confirmer, operating a conjugate transpose for the orthogonalvector, and generates a post-processing weight.